ath10k: release conf_mutex if vdev_start() fails

I modified Michal's commit c930f744 ("ath10k: implement channel switching") to return
when vdev_start() fails, but forgot to release conf_mutex.

Found by coccinelle:

>> drivers/net/wireless/ath/ath10k/mac.c:2745:5-11: preceding lock on line 2663

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>